In an 18 year study of four thousand middle-aged to elderly men, those who took 50mg of beta-carotene on alternate days scored much better in tests of general cognition and verbal memory than those who took a placebo.

abstract  :  A Randomized Trial of Beta Carotene Supplementation and Cognitive Function in Men: The Physicians' Health Study II.Grodstein F et al., Arch Intern Med. 2007 Nov 12;167(20):2184-90.

Ref: http://www. food for the brian.org